Risks owing to undesired static electricity or noises damaging internal electrical circuits are noticed recently in the field of mobile communication equipment such as cellular phones or the like.
A measure is disclosed in Japanese Patent Laid-Open Application No. 2001-127663 that has high pass filter 3, consisted of capacitor C0 and inductor L0 connected between antenna terminal 1 and switching circuit 2 to protect switching circuit 2 as shown in FIG. 8.
Mobile communication equipment such as cellular phones or the like are now making a rapid progress for downsizing year by year, which in this context requires downsized devices to house at inside of the equipment.
Above-mentioned high pass filter 3 must be altered to have a multiple stage form of capacitor C0 and inductor L0 to provide high pass filter 3 with a larger out-of-pass band attenuation. However, disadvantages must be agreed to accept not only an increased insertion loss but also a large device in size when a multiple stage form of capacitor C0 and inductor L0 is introduced to provide high pass filter 3 with a large out-of-pass band attenuation.
Additionally, if high voltage noises which have close frequencies to signal pass band and therefore can travel through high pass filter 3 intrude into, for example, mobile communication equipment, the static electricity or the high voltage noises will intrude directly into switching circuit 2 connected to an antenna terminal 1 to cause an incident of destroying the mobile communication equipment.
The present invention aims at providing the high-frequency devices capable of protecting circuit devices connected to post stages of antenna terminal 1 from possible high voltage noises having close frequencies to the signal pass band.